Abstract:

Purpose: During development, the presence of strabismus and anisometropia frequently leads to amblyopia, a visual disorder characterised by interocular acuity differences. Though additional deficits in contrast sensitivity, crowding (the impaired recognition of closely-spaced objects), and stereo-acuity are common, the relationship between these abilities is unclear. Method: We measured the co-variation between these four abilities in children aged 4-9 (n=72) with strabismus, anisometropia, or mixed strabismus/anisometropia, and unaffected controls. Children reported the orientation of a target (a modified "Pac-Man", similar to Landolt-C stimuli) using four "ghosts" as references. Using a modified staircase procedure we measured threshold size (acuity), contrast-detection, foveal crowding (the minimum separation between target and ghost-flankers supporting accurate identification), and stereo-acuity (with random-dot stereogram ghosts). Results: Group averages revealed significant interocular differences (IODs) in acuity for all three clinical groups (0.2-0.3 log minutes), and significant crowding IODs for the strabismic and mixed groups (0.6 and 0.4 degrees, respectively). Nonetheless, crowding IODs were correlated with acuity IODs in all four groups (r values between 0.43-0.59 and p<.05, albeit p=0.07 in the mixed group). Similarly, the occurrence of stereo-blindness (most common in strabismic and mixed groups) was associated with a significant increase in IODs for both acuity and crowding (each p<.05). No correlations were found with contrast detection. Conclusions: Our results demonstrate an association between IODs in acuity and crowding, and furthermore between these IODs and the presence of stereo-vision. We suggest that the deficits derived from strabismus and anisometropia lay along a continuum with the abilities observed during normal development.

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